1. Field of the Invention
The present invention relates generally to voltage regulation, particularly to a reference voltage generator for restraining the fluctuation of a reference voltage caused by temperature variations in an integrated circuit utilizing a low power supply voltage.
2. Description of the Prior Art
It is well known in the art that various integrated circuits utilize a reference voltage generator for the purpose of providing a reference voltage to circuit elements which detect or amplify an information signal. Demands made upon the reference voltage are basically that it remains stable even under circumstances of temperature variation and varied power supply voltages. Specially, in low power integrated circuits for reduced power dissipation, the reliability of the reference voltage becomes even more important than before.
FIG. 1 shows a conventional reference voltage generator which is disclosed in Korean Patent Publication No. 93-3927, published on May 15, 1993 (corresponding to Korean Patent Application No. 90-11946 filed by TOSIHBA CO. on Aug. 3, 1990). In the circuit of FIG. 1, the base and collector of an NPN bipolar transistor Q3 are coupled to the base of a PNP bipolar transistor Q2, and the emitter of transistor Q2 is connected to ground through a resistor R2. The collector of transistor Q2 is connected to the emitter of an NPN bipolar transistor Q1 through a resistor R1, and the emitter of transistor Q1 is also connected to the collector of transistor Q3 through a resistor R3. The base of transistor Q1 is coupled to a power supply voltage Vcc through a resistor R4 and is also coupled directly to the collectors of an NPN bipolar transistor Q4 and a PNP bipolar Q5, collector of transistor Q1 is connected to the power supply voltage Vcc. The base and emitter of transistor Q4 are coupled to the collector of transistor Q2 and to ground, respectively. The emitter of transistor Q5 is connected to ground through a resistor R5, and a battery VBB is connected between the base of transistor Q5 and ground.
The voltage difference .DELTA.VBE between the base and emitter of transistor Q2 (or of transistor Q3) across the resistor R2 is defined below: ##EQU1## where VBE2=voltage between the base and emitter of transistor Q2;
VBE3=voltage between the base and emitter of transistor Q3; PA0 I2=collector current of transistor Q2; and PA0 I3=collector current of transistor Q3. PA0 .DELTA.VBE4=voltage difference between the base and emitter of transistor Q4; and PA0 VT=volt-equivalent of temperature.
Thus, reference voltage Vref appearing at the collector of transistor Q4 may be given by: EQU Vref=(R1/R2).DELTA.VBE+.DELTA.VBE4=(R1/R2)ln(R1/R3)VT+.DELTA.VBE4 (2)
where
Since the VT has a positive factor proportional to temperature while VBE has a negative factor, the reference voltage Vref having a temperature coefficient of zero may be generated as a constant voltage by means of adjusting the resistance values of R1 to R3.
Although the conventional reference voltage generator as shown in FIG. 1 is able to provide a constant reference voltage, the conventional circuit can not be adapted to a system employing a power supply voltage of 1.5 V or less supplied by one battery or less because the constant voltage having a zero temperature coefficient appears at a voltage level of 1.2 V through 1.3 V and such range of the voltage level can not be provided until the power supply voltage is at least more than 2 V, taking into account the voltage drop between the base and emitter of transistor Q1. For meeting the demands of the lower power operation of integrated circuits such as memory devices, due to these limitations, the conventional reference voltage generator may not be capable of providing reliable operation of the low power device.